To get the process "un-stopped" and running again, we can bring it to the foreground with the bash built-in fg:

$ fg

^Z
[1]+  Stopped                 xeyes -center red
$

Now continue it in the background with the bash built-in bg:

$ bg
[1]+ xeyes -center red &
$

Great! The xeyes process is now running in the background, and we have a new, working bash prompt.